February 2025 Volume 7

MAINTENANCE

be good filtered city water or better. Most dry towers can operate without trim use in winter, spring, and fall and during summer nights when ambient temperatures are cooler. Closed-Circuit Evaporative Cooling Towers Closed-circuit pressurized cooling towers are used extensively to cool plant process water using a closed pressurized recir culating system. Scale and corrosion formation can be greatly reduced with a closed tower compared with an open type tower. Eliminating scale and sludge formation is critical in the water-cooling bundles (tubes). Use of a closed circuit tower prevents the water in the tubes from coming into contact with the spray water or at-mosphere air flowing across the tubes. Evaporative coolers can be shipped with factory-mounted controls that will cycle the fan and pump On/Off as needed to maintain the desired temperature set point based on the geographic location where it will be used. The system may still require some field adjustments. The outlet plumbing from the cooling tower will have two temperature sensing bulbs installed to control the outlet temperature of the tower. One bulb will be set to turn on the spray pump about 10°F below the required equipment inlet water tempera ture. The second bulb is set to turn on the blower motor about 5°F below the required equipment inlet temperature. The blower will then remain on until the water drops to a temperature less than 5°F below the required equipment inlet temperature. The spray pump will stay on until the water temperature drops to 10° F below the required equipment inlet water tempera ture. In a closed-circuit pressurized tower, a thermostatically controlled electric heater in the spray water sump pan turns on at a temperature of approximately 40°F to prevent freeze up in cold weather (Figs. 6 and 7). A float-operated valve controls the sump water level. These systems should have bleed-off controls to add enough water to overflow and allow scale and calcium to escape the tank.

Fig. 5: Air-cooled tower that uses an atomized mist assist option. Courtesy Dry Coolers Inc., Oxford, Mich.

Air-Cooled Heat Exchangers with Trim Cooler These systems use a dry-type tower with a temperature-regulated source of additional cooling (Fig. 4). The trim is only used when the air-cooled tower cannot reduce the water temperature low enough; usually in the summer during the afternoon hours when the ambient temperature exceeds the input temperature of the power supply by 10°F. the induction power supply will fault on a “hot-water input” and shut off. The additional trim can be a plate type heat ex-changer with city/well water cooling the dirty side of the heat exchanger and down the drain. Another type of trim is a compressor/chiller with the same type of plate or tube and shell heat exchanger. The trim atomized mist assist shown (Fig. 5) is only needed on extremely hot days in the summer when temperatures reach 90°F. The water used in the mist assist should

Fig. 6: Counter-flow closed-loop pressurized evaporative cooling tower; requires a pan heater.

Fig. 7: Schematic of a closed-loop pressurized evaporative cooling tower.

FIA MAGAZINE | FEBRUARY 2025 32